Germanwings currently operates CRJs as well, easyjet operated for a large part of their history two types as well.
Easyjet operated 2 types because of legacy and acquisition rather than a conscious decision to do so, in moving to an Airbus fleet the old fleet would over time move out, the acquisitions of Go and GB Aiways also gave them fleet issues. These were resolved quickly.
Air Berlin has not shown to have the same focus in doing this............. just means different engineers, different pilots and different sets of spares and parts.